In a properly designed p-n junction solar cell, the electrons move to the metal electrode which contacts the n-type silicon, and the holes move to the metal electrode which contacts the p-type silicon. These contacts are vitally important to the performance of the cell, since forcing current across a high resistance silicon/metal interface or through a high resistance electrode material robs useful power from the cell. The total specific series resistance of the cell, including interfaces and electrode material, should be no more than 1 .OMEGA.-cm.sup.2.
The need for a low-resistance contact places a fairly demanding requirement on the concentration of dopant atoms at the surface of the semiconductor. For n-type silicon, this dopant concentration must be .gtoreq.1.times.10.sup.19 atoms/cm.sup.3 (which is 200 parts per million atomic (ppma) based upon a density for silicon of 5.times.10.sup.22 atoms/cm.sup.3). For p-type silicon the requirement is less severe, with a surface concentration .gtoreq.1.times.10.sup.17 atoms/cm.sup.3 (2 ppma) being required. Furthermore, to maximize the electrical efficiency of a solar cell, it is often desirable to have a lower surface doping concentration everywhere except directly beneath the metal electrode.
Thus, it is desirable for a contact material to possess the following properties: an ability to supply a liberal amount of dopant to the silicon immediately beneath it (also known as self-doping), has high electrical conductivity, makes a mechanically strong bond to the silicon, does not degrade the electrical quality of the silicon by introducing sites where electrons and holes can be lost by recombination, inexpensive and lends itself to being applied by an economical process (such as screen printing).
A known contact material which possesses, to a significant extent, the above-described desirable properties, is aluminum. Aluminum possesses these properties when used for contacting p-type silicon and therefore forming the positive electrode in a silicon solar cell. This is due to the fact that aluminum itself is a p-type dopant in silicon. Aluminum can dope silicon, as part of a process which alloys the aluminum with the silicon, provided the processing temperature exceeds the aluminum-silicon eutectic temperature of 577.degree. C. The aluminum-silicon phase diagram is given in FIG. 1. The vertical axis of FIG. 1 is temperature in degrees centigrade, while the horizontal axis is percentage silicon. The horizontal axis has two scales: a lower scale of percent silicon (atomic) and an upper scale of percent silicon (by weight). FIG. 1 indicates a eutectic point 102 at 577.degree. C. with 12.5% Si and 87.5% Al (by weight). Line 100 indicates 577.degree. C. and therefore eutectic point 102 lies on this line. While the numbers directly below point 102 indicate 11.3% (atomic) and 11.7% (by weight) of Si at the eutectic, the more accurate data of detail graph 101 indicates that it is 12.1% (atomic) and 12.5% (by weight). It can be seen from curve 103 (which rises rightwards from point 102), that as the temperature further increases above the eutectic the percentage of Si, which can be held in a molten mixture of Si and Al, also increases. The alloying of aluminum and silicon at a temperature above the eutectic temperature produces: (i) a near-surface silicon region which is adequately doped with aluminum for low contact resistance, (ii) an electrode material having the aluminum-silicon eutectic composition which has a sufficiently high electrical conductivity for carrying solar cell currents, and (iii) excellent adherence between the eutectic conductor and the silicon substrate. Aluminum is also an inexpensive material which can be applied by screen printing using commercially available pastes.
For conventional solar cell structures the lack of a material, comparable to aluminum, for contacting n-type silicon in order to form the negative electrode of a solar cell, also makes the fabrication of a simple, cost-effective solar cell difficult. In a conventional solar cell structure, with a p-type base, the negative electrode (which contacts the n-type emitter) is typically on the front (illuminated) side of the cell and the positive electrode is on the back side. In order to improve the energy conversion efficiency of such a cell, it is desirable to have heavy doping beneath the metal contact to the n-type silicon and light doping between these contacts. One means that has been employed to accomplish this, termed "emitter etch back," is to begin with heavy doping over the entire front surface, and then to etch away part of the emitter after the contact metal has been applied. Such an etch-back process is time-consuming and difficult to control, since the heavily-doped layer is usually only .apprxeq.0.3 .mu.m deep.
The "emitter" is known in the solar cell art to be a thin layer that is doped in order to create a rectifying (also known as "p-n") junction capable of generating electrical current upon illumination. The "base" is that region which forms the other half of the p-n junction and is therefore doped to be of a semiconductor type opposite to that of the emitter. The base extends from the boundary of the emitter region to those contacts making ohmic electrical contact to the base. The emitter is thin with respect to the base.
Thus, the conventional silicon solar cell structure presently suffers from either increased processing complexity (to remove the n.sup.+ layer) or a loss of performance (if the n.sup.+ layer remains) because of the opposing demands for high doping density beneath the contact metal and low doping density between the contact metal areas.